For many years, it has been known that titanium dioxide can be used as a photocatalyst. A great deal of research has been done with a view toward providing photocatalytic coatings that have self-cleaning properties. The pursuit of self-cleaning photocatalytic window coatings, in particular, has been an active field of exploration. Such coatings typically involve a titanium dioxide layer carried by a glass pane. These coatings are commonly provided with a relatively thick layer of titanium dioxide and/or a specific under-layer system designed for achieving high levels of photoactivity. Thick titanium dioxide layers, unfortunately, produce high levels of visible reflectance, thus creating a somewhat mirror-like appearance. This high visible reflection tends to exaggerate the appearance of dirt on a window. Further, known under-layer systems commonly teach that specific materials and crystal structures must be used for the under-layer film(s) to achieve acceptable photoactivity levels. Moreover, many photocatalytic coating systems teach that heating is required during or after film deposition to achieve acceptable levels of photoactivity.
Known photocatalytic coatings also tend to have properties that are less than ideal for applications in which the coatings are used on windows. As noted above, the visible reflectance of many known photocatalytic coatings is unacceptably high. Moreover, the reflected colors of these coatings tend not to be ideal. Further, some of these coatings have particularly high surface roughness, as they are designed to have large surface areas that facilitate high photoactivity levels. These rough coatings, unfortunately, tend to be quite vulnerable to being abraded. They are also particularly susceptible to taking on and stubbornly retaining dirt and other contaminants, due to their high surface roughness. Finally, with many recent photocatalytic coatings (e.g., those in which complex under-layer systems are used to maximize photoactivity), it is unclear whether these coatings will exhibit the longevity (e.g., in-field durability over time) that is required for number-one-surface coatings.
The present invention provides low-maintenance coatings that offer exceptional durability, exceptional optical properties, reliable production processes, and surprising cleanliness/maintenance properties.